The existing polyolefin microporous film has been widely used as various battery separators, filters for separation, separator (film) for microfiltration, and the like, due to chemical stability and excellent mechanical properties. Among them, a porous film for a secondary battery requires high level of quality together with a need for stability of a battery. Recently, in accordance with the trend of high capacity and high output of the secondary battery, the need for improving properties of a porous film such as high strength, high permeability, heat stability of the porous film, and electrical stability of the secondary battery at the time of charging and discharging has further increased.
A lithium secondary battery requires high mechanical strength and heat stability to improve stability at a process of manufacturing a battery and during the usage, and high permeability in order to improve capacity and output. For example, when heat stability of the porous film is deteriorated, damage due to an increase in temperature in the battery may occur or short-circuit between electrodes may occur due to deformation, thereby causing overheat of the battery or increasing fire risk. In addition, as the utilization range of the secondary battery is expanded to automobiles for hybrid, it is important to secure stability of the battery in respect to overcharging, such that the porous film is required to have a property to withstand electrical pressure in respect to overcharge.
High strength is required to prevent loss of porous film that may occur in a process of manufacturing a battery or that may occur due to dendrites, and the like, produced in electrodes during a process of charging and discharging the battery to thereby prevent a short-circuit between the electrodes. In addition, when the strength of the porous film is weak at a high temperature, a short-circuit due to a film fracture may occur. After that, heat generation/ignition/explosion, and the like, due to a short-circuit between the electrodes occur. High strength may mean puncture strength in the present disclosure.
High permeability is required to improve capacity and output of a lithium secondary battery. In accordance with the trend requiring high capacity and high output of the lithium secondary battery, the need for a porous film having high permeability has increased.
Heat stability of the battery is affected by a shut down temperature, a melt down temperature, heat shrinkage, and the like, of the porous film. Among them, heat shrinkage at a high temperature has a significant influence on heat stability of the battery. In the case in which heat shrinkage is large, when an inner portion of the battery is at a high temperature, a portion of the electrode is exposed in a shrinkage process, which causes a short-circuit between the electrodes, and accordingly, heat generation/ignition/explosion, and the like, occur. Even though the melt down temperature of the porous film is high, when the heat shrinkage is large, a portion of the electrode may be exposed in a process of raising a temperature on the porous film, which may cause a short-circuit between the electrodes.
Japanese Patent Laid-Open Publication No. 1999-322989 (Patent Document 1) discloses a method of reducing shrinkage of a porous film. This method includes stretching a film in a machine direction only or reducing a total stretching ratio in order to reduce heat shrinkage in a transverse direction, such that an effect of improving physical properties obtained by the stretching process may not be obtained, such that the porous film manufactured by the method does not have excellent physical properties. In addition, products manufactured accordingly have significantly low puncture strength of about 0.06˜0.11N/μm at room temperature, such that it is difficult to improve stability of the battery.
As described above, in the existing porous film, high strength, high permeability, and low heat shrinkage required for improving stability of a high capacity/high output secondary battery may not be implemented, and at the time of manufacturing a multilayer composite porous film by applying a heat resistant resin and/or inorganic particles in order to improve heat shrinkage property, there are problems in that productivity is significantly deteriorated and cost considerably increases.